1. Field of the Invention
The present invention relates to a control device for a vehicle automatic transmission comprising two hydraulic coupling elements belonging to a first group and two or more hydraulic coupling elements belonging to a second group and adapted to selectively establish a plurality of shift ranges through a simultaneous engagement of the two hydraulic coupling elements of the first group and a simultaneous engagement of either one of the two hydraulic coupling elements of the first group and any one of the two or more hydraulic coupling elements of the second group.
2. Description of the Related Art
Conventionally, as a control device of this kind, a control device is known in which a shift control valve is provided for each of the hydraulic coupling elements of the first and second groups so that supplying and draining of oil to and from the respective hydraulic coupling elements are individually controlled by the respective shift control valves. In this control device, however, when the shift control valves malfunction, there may be caused a problem that oil happens to also be supplied to the other hydraulic coupling elements than the hydraulic coupling elements that are being intended to establish a desired shift range, resulting in a double meshing in which a plurality of shift ranges are established at the same time.
Here, in the vehicle automatic transmission in which a plurality of shift ranges are selectively established through a simultaneous engagement of two hydraulic coupling elements of the first group and a simultaneous engagement of either one of the hydraulic coupling elements of the first group and any one of the hydraulic coupling elements of the second group, however, as long as the shift control valves do not malfunction, there happens no case in which oil is supplied to three hydraulic coupling elements simultaneously. Japanese Patent Unexamined Publication No. Hei. 8-42681 discloses a conventional control device in which a fail-safe valve is interposed between shift control valves other than those corresponding to hydraulic coupling elements that are being intended to establish a predetermined single shift range and an oil supply passage communicating with a hydraulic pressure source, the fail-safe valve being constructed so as to freely be switched over between an open position where the oil supply passage is connected to the other shift control valves and a closed position where such a connection is disconnected. The fail-safe valve so structured is switched over to the closed position when hydraulic pressures of the respective hydraulic coupling elements of first and second groups are inputted into the fail-safe valve to be found that any three of those hydraulic pressures increase. According to this construction, oil is prevented from being supplied to the hydraulic coupling elements corresponding to the other shift control valves, a double meshing being thereby prevented.
In the conventional example, there is a need to form individual oil chambers into which hydraulic pressures of the respective hydraulic coupling elements of the first and second groups are inputted individually, and when the total number of hydraulic coupling elements is increased to meet an increase in the number of shift ranges, the number of oil chambers is also increased. As a result of this, the length of the fail-safe valve is increased, and there is increased the possibility that a valve stick is caused. Moreover, the diameter of the valve element of the fail-safe valve is made thicker, and the mass of the valve element is increased, this deteriorating the response of the fail-safe valve when switched over from the open position to the closed position.